1. Field of the Invention
The present invention relates generally to implantable cardioverter defibrillators and, more specifically, to an implantable cardioverter defibrillator employing polymer thin film capacitors.
2. Background of the Invention
The incidence of heart disease in the United States is significant with approximately one out of every two deaths is attributable to heart disease. One of the leading complications secondary to heart disease is cardiac arrythmia resulting in sudden cardiac death. Because of the high prevalence of sudden cardiac death due to cardiac arrythmia there is a demonstrated need for an implantable cardioverter defibrillator (ICD) system. To be useful, an ICD systems must be self-contained, complete, and capable of effective repetitious function autonomous from the outside world. A direct corollary of these requirements is that the system be small enough to be implanted. The size limitations imposed by an implantable device generally have prevented the technology and techniques applicable to external defibrillator systems from being applied to ICD systems. Even with the development of novel technology and techniques for generating defibrillation pulses or countershocks in an implantable device, present day ICD systems are still sufficiently large so as to require implantation within the abdomen or abdominal wall of a patient.
A more ideal size for an ICD system would be the size that implantable cardiac pacemakers have achieved, such that the device is capable of being implanted in the subcutaneous space just inferior to either clavicle. Unfortunately, pacemakers are capable of smaller sizes quite simply because their power requirements are significantly less than the power requirements of ICD systems. While pacemakers have energy output requirements that are in the microjoule output range, ICD systems, to be effective, must be capable of delivering repeated defibrillation countershocks above at least about 15 joules for each countershock. In order to achieve such high energy outputs, existing ICD systems have utilized larger batteries, as well as large high voltage capacitors to generate the required high energy defibrillation countershock. Consequently, one of the major challenges in reducing the size of an ICD system is how to decrease the effective size limits of both the batteries and capacitors that are used by the ICD system.
Presently, there are three different types of ICD systems which have received device approval from the Federal Drug Administration, the PCD.TM. device, available from Medtronic, Inc., of Minneapolis, Minn., the Cadence.RTM. device, available from Ventritex, Inc., of Mountain View, Calif., and the Ventak-P.RTM. device, available from Cardiac Pacemakers, Inc., of St. Paul, Minn. The primary components of all existing ICD systems include an automatic monitoring and detection mechanism, a capacitor system, a battery system and control circuitry for detecting a ventricular arrhythmia and controlling delivery of a high-voltage capacitive-discharge electrical countershock in response by charging and then discharging the capacitor system. To achieve successful defibrillation, the ICD system must deliver a high voltage electrical countershock with an initial voltage of greater than about 500 to 600 volts.
The existing ICD systems are all capable of delivering a maximum countershock of up to 700 to 750 volts having a total energy of between 31 to 44 joules. At the time an ICD system is implanted in a patient, the attending physician will empirically determine a minimum defibrillation threshold for the patient, and will program the charging voltages for the countershocks to be delivered as part of a therapy regimen within the range of maximum voltages allowed by the device. In addition, the attending physician can also typically program when the electrical countershock is to be truncated by programming the duration of the countershock in a range from about 6 to 9 milliseconds. Alternatively, the duration can be altered by programming the initial discharge voltage of the ICD system and allowing the tilt of the ICD system to establish the point at which the countershock will be truncated. The tilt of an ICD system is defined as the percentage decline in the output voltage from the charging voltage to the voltage at the time the discharge is truncated. For existing ICD systems, the tilt is typically set at about 65%.
The capacitor system is a critical element of the ICD system, both in terms of how effective the ICD system can be and how small it can be. By definition, a capacitor is comprised of two conductive surfaces separated by an insulating material. The insulating material is known as the "dielectric" of the capacitor. When the two surfaces of the capacitor are oppositely charged by a voltage source, such as the battery in an ICD system, electrical energy is effectively stored by the capacitor in the polarized dielectric. The capability of a capacitor to store electrical charge is the capacitance value of the capacitor. For a given dielectric material, the thinner the dielectric, the higher the capacitance value. A thinner dielectric also decreases the overall size of the capacitor. Unfortunately, there are limits as to how thin a dielectric can be due to the fact that very thin dielectrics will break down under high voltages as there is simply an insufficient amount of insulating material between the conductive surfaces to withstand the high voltages.
When all of these requirements for an ICD system are considered, the aluminum oxide electrolytic capacitor has proven to be the best capacitor technology for use in ICD systems to date, and is presently used in all existing ICD systems. The aluminum oxide dielectric can be made very thin because the dielectric oxide is essentially grown on the conductive surface of a very thin sheet of aluminum that has been etched to increase its effective surface area. As a result, aluminum oxide electrolytic capacitors have higher energy densities (typically 1.7 to 1.8 joules/cc) than other types of capacitor technologies (typically much less than 1.5 joules/cc). Due to the nature of the aluminum oxide dielectric, however, electrolytic capacitors are limited to maximum rated charging voltages in the range of approximately 350 to 375 volts. Beyond 375 volts, electrolytic capacitors begin to suffer from significant leakage current across the dielectric. This leakage current increases rapidly as the voltage is increased and charging of the electrolytic capacitor will cease when the leakage current equals the charge current. As a result, the existing ICD systems all utilize two electrolytic capacitors in series, each being charged to approximately 350 to 375 volts, which are then discharged to deliver the high voltage shock to the myocardium having a maximum voltage of approximately 700 to 750 volts.
Although electrolytic capacitors are used in existing ICD systems in order to take advantage of their excellent capacitance to volume ratio, electrolytic capacitors suffer from several major drawbacks. First, the useful charging voltage for electrolytic capacitors is limited to approximately 350 to 375 volts due to the current leakage effects encountered at higher charging voltages. Electrolytic capacitors, due to the nature of the oxide dielectric, begin to break down and suffer from significant current leakage with charging voltages over 375 volts. Because and ICD system needs to produce an initial discharge voltage of at least about 600 volts, this drawback requires that two electrolytic capacitor be used in series in order to generate the required initial discharge voltage, thus increasing the number of components within the ICD system and, to a certain extent, complicating the electronic design of the ICD system.
Another significant disadvantage of electrolytic capacitors is the degradation of the oxide dielectric over time. The oxide can be reformed by periodic charging to full voltage. On a monthly or quarterly basis, the capacitor system will need to be charged to its full voltage. In early ICD systems, this requirement necessitated the patient's periodic return to the hospital to accomplish the reforming of the capacitor system. Later ICD systems have used automatic reforming of the electrolytic capacitors from the internal battery system on a periodic basis, but this practice is wasteful of valuable energy in an ICD system that only has a finite and depletable source of power.
Still another drawback of electrolytic capacitors is that a substantial portion of the energy density advantage over other capacitor technologies is lost to packaging inefficiencies within the ICD system as a result of the cylindrical packaging shape that is required of electrolytic capacitors. When the lost volume of fitting a cylindrical volume into a rectangular volume is factored into the energy density calculations, the energy density for electrolytic capacitors is effectively only about 1.3 to 1.4 joules/cc.
At least one other capacitor arrangement has been proposed for use in an ICD system. In U.S. Pat. No. 3,614,954 issued to Mirowski, the device described charges a capacitor of an unspecified capacitor technology to a charging voltage of 2,500 volts in order to generate approximately 50 joules of energy for the electrical countershock. At the time the Mirowski patent application was filed, the available capacitor technologies that had breakdown voltages of greater than 2,500 volts were probably mica or oil-soaked paper dielectric capacitors. For a number of reasons, and regardless of the capacitor technology utilized, the capacitor arrangement suggested by Mirowski would not be suitable for use in a practical ICD system, and, in fact, no ICD system has ever been developed using a capacitor charged to such a high voltage. The primary reasons for this are the unacceptably high peak currents that would be generated by an electrical countershock of such a high voltage, the excessively high voltages which prevent use of transistor switches and the large size of the capacitor and battery system required to charge and store 50 joules of energy.
With respect to the first reason, myocardial tissue resistance between any two implanted discharge electrodes has been found to be about 50 ohms (.OMEGA.) on average. This value of 50 ohms has become the accepted average resistance of the myocardial tissue between the discharge electrodes for ICD systems. Using this average resistance value, the peak current of an electrical countershock delivered from a capacitor charged to 2500 volts would be 2500 volts/50 ohms, or 50 amperes. It is known that peak currents in excess of about 30 amperes during delivery of an electrical countershock can lead to tissue destruction in the heart in a zone beginning from the center of the electrical field and extending outward. High peak currents also stun tissue extending radially outward from the border of the destruction zone for some additional distance. For additional background on this type of high current tissue destruction, reference is made to: "Alterations Induced by a Single Defibrillation Shock Applied through a Chronically Implanted Catheter Electrode" by Barker-Voelz et al. in J. Electrocardiology 16(2): 167-180, 1983.
With respect to the second reason, most microelectronic switching circuitry have maximum switching voltages of less than about 1000 volts. Certain types of microelectronic switching circuitry, such as silicon controlled rectifiers or certain high power MOSFET transistors are capable of handling switching voltages of up to about 2000 volts. To date, however, no microelectronic switching circuits have been developed which could handle the 2500 volts required by the Mirowski device. Thus, the Mirowski device requires the use of individual high power switching transistors, devices which occupy significantly more space in the device than microelectronic switching circuitry.
As for the third reason, an analysis of just the stored energy requirements of the Mirowski device and existing ICD systems reveals that the Mirowski device, which stores 50 joules of energy, would require a battery and capacitor system that are at least 15% to 40% larger than the battery and capacitor systems of existing ICD systems, which store maximum energies of between about 31 to 44 joules. While it might seem possible to lower the maximum stored energy of the Mirowski device to that of existing ICD systems, the maximum stored energy requirements of the Mirowski device effectively cannot be lowered because to do so would decrease the overall duration of the defibrillation countershock below about 2 milliseconds, a point below which defibrillation effectiveness decreases significantly. Thus, it is not practical to decrease the size of the Mirowski device by decreasing the stored energy requirements below the 50 joules taught in the Mirowski patent.
A potential alternative capacitor technology to electrolytic capacitors is the polymer thin film capacitor. In a polymer thin film capacitor, the dielectric is a very thin polymer film that is formed mechanically through high precision rolling operations. Conductive layers of aluminum are then deposited on each surface of the polymer film. The advantages of the polymer thin film capacitors are that they have very high breakdown voltages and very good charge retention. As a result, if a polymer thin film capacitor were used in an ICD system, there would be no need to use two separate capacitors to achieve the initial discharge voltage required for defibrillation. Nor would there be a need to reform the capacitor due to a break down of the dielectric.
To date, however, polymer thin film capacitors have not been suggested for use in an ICD system for several reasons due to the nature of the polymer thin film dielectric. First, polymer thin film capacitors have lower average energy densities than electrolytic capacitors because the polymer thin film dielectric is substantially thicker than the aluminum oxide dielectric. Second, polymer thin film capacitors operate more efficiently at higher voltages and, hence, the energy density in the voltage range of 700 to 750 volts, for example, is even less efficient.
Although a single polymer thin film capacitor can withstand significantly higher voltages than an electrolytic capacitor, polymer thin film capacitors constructed to work in the 700 to 750 volt range and capable of storing 35 to 40 joules of energy will occupy substantially more volume than the two aluminum oxide electrolytic capacitors presently used in existing ICD systems. This size factor is directly related to how thinly the polymer film dielectric can be rolled. Aluminum oxide dielectric is approximately 1.5 nanometers (nm) thick per rated charge volt. Therefore, an electrolytic capacitor rated at 350 volts will have a dielectric thickness of approximately 525 nm. In contrast, polymer thin films simply cannot be rolled any thinner than about 1,000 nm due to the fact that pin hole defects and impurity domains capable of causing catastrophic breakdown within the polymer thin film dielectric increase dramatically at thicknesses below about 1,000 nm. In practice, most manufacturers of polymer thin film capacitor stay nearer to the 10,000 nm thickness range for the polymer thin film dielectric in order to avoid catastrophic breakdown secondary to pin hole defects and impurity domains. Because the capacitance value of a capacitor is a function of the thickness between the conductive surfaces, a polymer thin film capacitor will have to have at least twice the surface area of an electrolytic capacitor in order to compensate for being at least twice as thick. Consequently, a polymer thin film capacitor would need to be at least twice the volume of its electrolytic counterpart. This is hardly an ideal situation for use in an ICD system, and, as a result, polymer thin film capacitors have not been suggested for use in ICD systems because of the comparatively enormous sizes required.
While the use of electrolytic capacitor for ICD systems has allowed for the creation of practical implantable devices that can deliver effective electrical countershocks, there are inherent limitations of electrolytic capacitors which hinder any further reduction in the size of ICD systems by reducing the size of the capacitor system necessary to deliver the capacitive-discharge electrical countershock. Unfortunately, none of the other capacitor technologies appear to offer a more practical alternative to the use of electrolytic capacitors for ICD systems. Therefore it would be desirable to provide an implantable cardioverter defibrillator system which could employ the use of a capacitor technology other than electrolytic capacitors. In addition, it would be advantageous to provide an implantable cardioverter defibrillator system that could take advantage of the higher charging voltages available with polymer thin film capacitors.